1. Field of the Invention
The present invention relates to an optical integrated device for a reproducing head for magneto-optical record.
2. Related Background Art
Recently, efforts have been made to develop an optical recording method which satisfies various requirements including high density, high capacity, high accessing speed and high recording/reproducing speed.
Among various optical recording methods, magneto-optical recording is most attractive because it allows erasure after recording (reproducing) information and repeated recording of new information.
A record medium used in the magneto-optical recording has a single-layer or multi-layer vertical magnetization film as a record layer. The magnetization film may be made of amorphous GdFe, GdCo, GdFeCo, TbFe, TbCo or TbFeCo. The record layer usually has concentric or helical tracks on which information is recorded.
The information to be recorded is previously binarized and it is recorded by two signals, one bit having upward magnetization to the film plane and the other bit having downward magnetization. Since those bits correspond to the digital signals "0" and "1", the former is called "0" bit and the latter is called "1" bit.
However, usually, the magnetization of the track on which the information is to be written is aligned to "upward", for example, by applying a strong external magnetic field prior to recording. This process is called an initialization. Thereafter, the "1" bits having the downward magnetization are formed on the track. The information is recorded by the presence or absence and/or the bit length of the downward "1" bit.
An optical head used to reproduce the magneto-optical record has a complex structure in order to detect "0" and "1" bits. FIG. 1 shows a conceptual view of an optical head which uses a most typical differential detection system.
A laser diode 31 is usually used as a polarized light source. A laser beam emitted by the laser diode 31 has an oval light beam sectional plane as shown in FIG. 2(a) when it is taken along a line A--A' of FIG. 1. A polarization plane is parallel to a minor axis as shown by arrows in FIG. 2.
The laser beam is then directed to a collimator lens 32, which collimates the laser beam. The collimated beam is directed to a first prism 33 and a second prism 34. Those prisms function to shape the sectional shape of the beam from oval to real circle. They are arranged in a predetermined attitude. The sectional plane and the polarization plane of the beam transmitted through the prisms 33 and 34 are shown in FIGS. 2(b) and 2(c), respectively. As seen from the figure, the polarization plane is not rotated.
The shaped beam is then directed to a main beam splitter 35, which guides light reflected by a record medium (MO) to a detecting optical system.
The beam transmitted through the beam splitter 35 is focused by an objective lens 36 and directed to the record medium (MO).
The directed beam is reflected by the record medium (MO) to go back along the same path. However, the reflected light has the polarization plane rotated by an angle .theta..sub.k. (This phenomenon is called a Kerr effect.) The rotation of the polarization plane is clockwise (+.theta..sub.k) or counterclockwise (-.theta..sub.k) depending on whether the beam is directed to the "0" bit (upward magnetization) or "1" bit (downward magnetization). This is shown in FIGS. 3(a) and 3(b), whether the rotation is +.theta..sub.k or -.theta..sub.k depending on the upward magnetization or downward magnetization depends on the type of magnetic material. (In FIG. 3, -.theta..sub.k for "0" bit and +.theta..sub.k for "1" bit.)
The beam reflected by the medium again passes through the objective lens 36 and is directed to the main beam splitter 35 where it is split into two parts, one being directed to the light source 31 and the other to the detecting optical system.
The beam directed to the detecting optical system is directed to a one-half wavelength plate 37 which has an optical axis thereof inclined by 22.5 degrees to the polarization plane of the incident light. As a result, the polarization plane is rotated by .alpha.=22.5.times.2=45 degrees. The angle 45 degrees is the value used in a most conventional method called a 45 degrees differential method. In an asymmetric differential method, the angle .alpha. is set to 10.about.15 degrees. The rotated beams are shown in FIGS. 3(c) and 3(d).
The beam is then directed to a polarization beam splitter 38 where it is split to a P component and an S component. The S component of a light vector is shown by solid line arrows in FIGS. 4A and 4B. Broken line arrows show the polarization direction of the beam before the rotation. It is seen that the magnitude of the S component changes depending on whether the beam is the reflected beam from "0" bit (FIG. 4A) or the reflected beam from "1" (FIG. 4B).
On the other hand, the P component of the light vector is shown by solid line arrows in FIGS. 5A and 5B. Broken line arrows show the polarization direction of the beam before the rotation. Again, the magnitude of the P component changes depending on whether the beam is the reflected beam from "0" bit (FIG. 5A) or the reflected beam from "1" bit (FIG. 5B).
One of the split S component and P component is then directed to a first detector 39 while the other is directed to a second detector 40, where they are converted to electrical signals, respectively.
The converted electrical signal is proportional to the square of the S component or P component. Accordingly, an output from the first photoelectric converter 39 or the second photoelectric converter 40 changes depending on whether the split component of the reflected beam from "0" bit is received or that from "1" bit is received. Accordingly, the information recorded on the record medium (MO) is reproduced in the form of an electrical signal.
Since the AC components of the outputs of the first photoelectric converter 39 and the second photoelectric converter 40 are of opposite phase, the AC output is doubled by differentiating both outputs and noise due to the fluctuation of the light source 31 is also eliminated. This is a principle of the differential method. The outputs of the first photoelectric converter 39 and the second photoelectric converter 40 are supplied to a differential amplifier (not shown).
The number of components of the reproducing head for the magneto-optical record is as many as ten for the optical head shown in FIG. 1, and hence problems of difficulty in reducing the size of the head, heavy weight, high manufacturing cost and time-consuming work for mounting and fixing the parts are encountered.
An optical integrated device for a monolithic reproducing head for a magneto-optical record in which all parts are formed on a single substrate has been proposed. For example, reference is made to FIG. 2 of Japanese Laid-Open Patent Application No. 1-134730.
However, many problems must be solved in putting the device into practice. In the optical integrated device, a crystal layer such as lithium niobate for forming a polarization optical system which utilizes an electro-optical effect and a GaAs crystal layer for forming a light source and a photoelectric converter are required. However, because of different crystalline structures of those crystals, it is not possible by the present technology to manufacture a substrate on which both crystals are stocked.